1. Field of the Invention
The present invention relates to a multiplex transmission system having a multiplex transmission path to which a plurality of multiplex nodes are connected.
2. Description of the Related Art
A so-called multiplex transmission system, in which data communication with a large number of control units and the like can be performed through one communication line, is generally known (e.g., Japanese Patent Laid-Open No. 1-143533). Recently, the following multiplex transmission system has been employed locally even in vehicles such as automobiles. According to this system, electronic devices, operation switches, and the like in a vehicle are connected to nodes connected to a multiplex transmission path, and data communication within the vehicle is performed through the nodes and the multiplex transmission path.
The above-described multiplex transmission system is designed to perform data communication between a large number of electronic devices through one communication path, to which a plurality of nodes are generally connected.
In such a multiplex transmission system, for example, each node which is not connected to the power supply is started by connecting it to the power supply. Alternatively, each node may not be started until the ignition key is turned on even if it is connected to the power supply. In this case, each node is started by turning on the ignition key. In either case, as the number of nodes connected to the multiplex transmission path increases, the time required to start the nodes is prolonged.
It is, therefore, required that the time taken to start the nodes at the start time of the system be minimized.
In some known multiplex transmission system, data is output not only at the occurrence of an event, e.g., a switching operation, but also at a predetermined period to always supply data indicating the states of the electronic devices, the switches, and the like connected to the respective nodes to the multiplex transmission path.
In such a type of system, attempts have been made to shorten the period of an output from each node at the start time of the system and restore the output period to the normal period after the system is stabilized. For example, Japanese Patent Laid-Open No. 4-129347 discloses a system in which information is transmitted between the respective control units at a predetermined period, and information is transmitted at a period shorter than the predetermined period at the start time of an engine.
As described above, by shortening the period of an output from each node at the start time of the system, the time required to start each node at the start time of the system can be shortened.
If, however, the period is excessively shortened, the traffic intensity of the multiplex transmission path is excessively increased. As a result, it takes much time to transmit/receive information, or necessary information may not be transmitted/received.